the Incomparably Shrewd and Clever
To clarify, when I talk about the system not working for 50%, I don't mean to imply that the system completely fails for them. I think for 50% (or maybe 60%, using the playtest's satisfaction threshold) of the groups out there, they use the 5e skill system out of the book and have no problem at all with it. For the other 40% to 50%, some may make small adjustments to the system to be happy with it, such as adding a skill or two, deleting a skill, changing availability of a skill to a class or background, or defining a skill or two somewhat differently from standard 5e, for example. For others, bigger changes are required. I think this is where the question about "how hard is hard" and what the DCs mean to people comes in, because this affects the system as a whole, it's a larger tweak.Then that is where I think you are wrong. Well, I'll certainly grant that the existing skill system won't work very well quite often (50% breakdown rate might be a bit higher than I would say though.) But I emphatically reject the idea that all possible alternatives are equally ineffective. That's patently ridiculous--testing and iterating should allow SOME improvement. We may not all think alike (as I argued above), but there are commonalities that can be leveraged. And if we can hit, say, only a 10% breakdown rate, that would be a HUGE improvement.
I guess it's the distinction between people who feel "dissatisfied," "mostly dissatisfied," and "completely dissatisfied" with the standard 5e skill system. To put a totally wild guess to those, I would say they are 20%, 15%, and 5%, respectively, making up the 40%. So there might be some of that 20% you can pull into the satisfied column, but you need to be careful that you don't lose a corresponding amount (or more) from the current "satisfied" column. I think the variety of campaign styles and tones, along with personal playstyle preferences of so many players makes this a tough job.
That is why I say present some optional components that cover a broader spread of those styles and preferences. Support the players who say "skills are too broad" and the players who say "skills are not specialized enough." Not with a hard mechanical rule system to adjust, but a discussion of the implications of changes, and what effects they might have on play, as well as guidelines on how to achieve certain common playstyles with the skill system.